The present invention relates to an image heating apparatus such as a thermal fixing device mounted in an image forming apparatus such as a copying machine, a printer, or the like. In particular, it relates to an image heating apparatus utilizing an induction heating principle.
An image heating apparatus such as a thermal fixing device makes up a large proportion of energy consumption in an entire image forming apparatus, so that the image heating apparatus is desired to reduce its power consumption. Further, there is also a large demand for a reduction in waiting time for printing.
As an image heating apparatus meeting such demands, an induction heating type image heating apparatus has attracted attention (e.g., Japanese Laid-Open Utility Model Application No. Sho-51-109739).
FIG. 16 shows the general structure of an example of an electromagnetic induction heating type fixing apparatus.
In the drawing, a reference numeral 10 designates a fixing film (which hereinafter will be referred to as a sleeve) comprising an electromagnetic induction type heat generating layer (electrically conductive layer, magnetic layer, electrically resistive layer). The fixing film 10 is cylindrical and flexible, and is used as a rotational heating member.
A reference numeral 16c designates a film guiding member (which hereinafter will be referred to as sleeve guiding member) in the form of a trough, which is approximately semicircular in cross section. The sleeve 10 is loosely fitted around the sleeve guiding member 16c. 
A reference numeral 15 designates a magnetic field (flux) generating means disposed within the sleeve guiding member 16c. The magnetic field generating means comprises an exciting coil 18, and a magnetic core 17 having an T-shaped cross section.
Designated by a reference numeral 30 is an elastic pressure roller, which is kept pressed upon the bottom surface of the sleeve guiding member 16c, with the interposition of the sleeve 10, with the application of a predetermined pressure, forming a fixing nip N having a predetermined width.
The magnetic core 17 of the magnetic field generating means 15 is disposed so that its position corresponds to the position of the fixing nip N.
The pressure roller 30 is rotationally driven by a driving means M, in the counterclockwise direction indicated by an arrow in the drawing. As the pressure roller 30 is rotationally driven, friction occurs between the peripheral surface of the pressure roller and the outwardly facing surface of the sleeve 10, in the fixing nip N. As a result, the sleeve 10 is rotated by the pressure roller 30, around the sleeve guiding member 16c, in the clockwise direction indicated by an arrow in the drawing, at a peripheral velocity substantially equal to the peripheral velocity of the pressure roller 30, with the inwardly facing surface of the sleeve 10 sliding on the bottom surface of the sleeve guiding member 16c, in the fixing nip N (pressure roller-driving method).
The sleeve guiding member 16c plays the role of maintaining the fixing pressure in the fixing nip N, the role of supporting the magnetic field generating means 15 comprising the combination of the exciting coil and magnetic core 17, the role of supporting the sleeve 10, and the role of keeping the sleeve 10 stable while the sleeve 10 is rotationally driven. The sleeve guiding member 16c is formed of such a material that does not prevent the passage of a magnetic flux through the sleeve guiding member 16c and that can withstand a large amount of load.
The exciting coil 18 generates an alternating magnetic flux as alternating current is supplied to the exciting coil 18 from an unshown exciting circuit. The alternating magnetic flux generated by the exciting coil 18 is concentrated to the fixing nip N, by the magnetic coil 17 with the T-shaped cross section disposed so that its position corresponds to that of the fixing nip N. The magnetic flux concentrated to the fixing nip N generates eddy current in the electromagnetic induction type heat generating layer of the sleeve 10. This eddy current and the specific resistance of the electromagnetic induction type heat generating layer generates heat (Joule heat) in the electromagnetic induction type heat generating layer. With the presence of the magnetic core 17 with the T-shaped cross section which concentrates the alternating magnetic field to the fixing nip N, the electromagnetic induction heat generation is concentrated to the portion of the sleeve 10 within the fixing nip N. Therefore, the fixing nip N is highly efficiently heated.
The temperature of the fixing nip N is kept at a predetermined level by a temperature control system, inclusive of an unshown temperature detecting means, which controls the current supply to the exciting coil 18.
Thus, as the pressure roller 30 is rotationally driven, the sleeve 10 is rotated around the sleeve guiding member 16, while current is supplied to the exciting coil 18 from the exciting circuit. As a result, heat is generated in the sleeve 10 through electromagnetic induction, increasing the temperature of the fixing nip N to a predetermined level, at which it is kept. In this state, a recording medium P, on which an unfixed toner image t has been formed, is conveyed to the fixing nip N, or the interface between the sleeve 10 and pressure roller 30, with the image bearing surface of the recording medium P facing upward, in other words, facing the surface of the fixing sleeve. In the fixing nip N, the recording medium P is conveyed with the sleeve 10, being sandwiched between the sleeve 10 and pressure roller 30, the image bearing surface of the recording medium P remaining flatly in contact with the outwardly facing surface of the sleeve 10. While the recording medium P is conveyed through the fixing nip N, the recording medium P and the unfixed toner image t thereon are heated by the heat generated in the sleeve 10 by electromagnetic induction. As a result, the unfixed toner image t is permanently fixed to the recording medium P. After being passed through the fixing nip N, the recording medium P is separated from the peripheral surface of the rotating sleeve 10, and then, is conveyed further to be discharged from the image forming apparatus.
Incidentally, as described above, the exciting oil 18 is required to approach the fixing sleeve 10. More specifically, as shown in FIG. 17, the exciting coil is, e.g., wound substantially in a planar shape and then transformed into a boat shape by bending it in a direction of arrows in the drawing (e.g., Japanese Laid-Open Patent Application (JP-A) No. 2000-243545).
A dimensional relationship in a longitudinal direction among the thus-prepared coil 18, the magnetic core 17, the sleeve 10, and the recording medium P is shown in FIG. 18.
Referring to FIG. 18, the magnetic core 17 is designed to have a length in its longitudinal direction substantially identical to that of the recording medium P. Further, the coil 18 has a longitudinal length longer than that of the magnetic core 17, and the sleeve 10 has a longitudinal length longer than that of the coil 18.
However, as shown in FIG. 16, the sleeve guiding member 16c functioning as a sliding surface (layer) with respect to the sleeve 10 in the nip N is present between the sleeve 10 and the magnetic core 18, thus resulting in a gap dxe2x89xa00. For this reason, at both end portions of the coil 18 in the longitudinal direction, a magnetic flux does not enter perpendicular to the sleeve 10. As a result, a region of action of the magnetic fluxes is narrowed to cause a temperature-lowering region at both end portions of the sleeve in comparison with a central portion thereof. As a result, as described above, when the longitudinal lengths of the recording medium P and the magnetic core 17 are set to be substantially identical to each other (FIG. 18), the recording medium P has caused fixation failure at end portions in some cases.
On the other hand, when the longitudinal length of the magnetic core 17 is made sufficiently larger than a width (longitudinal length) of the magnetic core 17 in order to suppress the occurrence of fixation failure, the following problems have arisen.
(1) Heat due to an eddy current is always generated also at a non-paper feeding region of the sleeve 10, and in the region, there is no heat removal by the recording medium. As a result, the sleeve 10 causes excessive temperature rise in the non-paper feeding region, thus being undesirably damaged.
(2) With the extension of longitudinal length of the magnetic core 17, the coil 18 is also required to be extended. However, if the longitudinal length of the coil 18 is made too large, heat dispersion from the end portions of the sleeve 10 is considerably increased, thus remarkably lowering a power efficiency.
The present invention has accomplished in view of the above-mentioned problems.
An object of the present invention is to provide an image heating apparatus capable of suppressing an occurrence of heating failure of an image.
Another object of the present invention is to provide an image heating apparatus capable of suppressing excessive temperature rise in a region through which a recording medium does not passes.
According to the present invention, there is provided an image heating apparatus for heating an image formed on a recording material, comprising:
a heating member having a heat-generating layer, and
magnetic field-generating means for generating a magnetic field to induce an eddy current in said heat-generating layer, said magnetic field-generating means comprising a core and a coil disposed around said core in a longitudinal direction of said core;
wherein said coil has a minimum length L1 and a maximum length L2 respectively in a longitudinal direction thereof; said core has a maximum length Lc in the longitudinal direction thereof; said core and said heating layer form a gap d therebetween; and the recording material has a prescribed maximum size giving a passing width Lp, satisfying the following relationship:
Lp+2(2d+1)xe2x89xa6Lcxe2x89xa6L1 less than L2.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.